Conventionally, as a pressure sensor detecting pressure from the amount of deformation of a diaphragm, there is a pressure sensor as disclosed in Patent Document 1. The pressure sensor according to the document has a structure in which a silicon substrate having the diaphragm functioning as a movable electrode and a glass substrate are bonded together and thereby a surface side of the diaphragm is sealed as a pressure reference chamber.
The diaphragm flexes or deforms due to the reflection of a pressure difference between an external pressure and a reference pressure in the pressure reference chamber under a use environment, whereby the diaphragm is in a state of contacting with a dielectric layer on the glass substrate. Under the dielectric layer of the glass substrate is formed an opposing electrode, so that a function as the pressure sensor is performed by detecting the contact area change dependent on the external pressure as a change of interelectrode capacitance. The pressure sensor having such a structure is called a touch-mode capacitive type pressure sensor and has high detectivity and high-voltage tolerance.
Patent Document 1 Japanese Unexamined Patent Application Publication No. 2002-214058
In the above-described pressure sensor, bonding of the silicon substrate and the glass substrate can be performed by anodic bonding. Compared with bonding using a bonding material (such as a wax material), the anodic bonding is suitable to appropriately adjust the reference pressure in the pressure reference chamber because it is not accompanied by generation of gas, as well as it is a method suitable to exactly define a gap between the movable electrode and the opposing electrode (dielectric layer).
However, anodic bonding is a technique requiring high flatness of bonded surfaces and has a problem that it is difficult to obtain a good bonding strength when the flatness is insufficient. Nevertheless, in the structure of the above-described pressure sensor, since a conductive wire for capacitance detection is led out so as to traverse a bonded region from the pressure reference chamber, a level difference equivalent to a film thickness of the conductive wire is produced on the bonded surfaces. Thus, it is difficult to perform a reliable sealing of the pressure reference chamber by anodic bonding.